1. Field of the Invention
The present invention relates to motor control devices equipped with switching elements to control the operation of a multi-phase AC motor such as a three-phase AC motor mounted onto a vehicle. In particular, the motor control device generates pulse width modulation (PWM) signals to turn the switching elements on and off in order to control the operation of the multi-phase AC motor.
2. Description of the Related Art
For example, Japanese patent laid open publication No. JP 2006-158126, as one of related-art techniques, discloses a motor control device equipped with a plurality of switching elements for controlling a three-phase AC motor mounted on a vehicle. The motor control device instructs the switching elements to turn on and off in order to control the operation of the three-phase AC motor. The motor control device shown in JP 2006-158126 is comprised of an inverter unit and a PWM signal calculation unit. The inverter unit is equipped with a switching circuit. The switching circuit is composed of three groups of MOS field effect transistors (MOSFETs) placed in parallel. Each group of MOSFETs corresponds to each phase of the three-phase AC motor. The MOSFETs in each group is connected in series.
The PWM signal calculation unit generates PWM signals and transmits the PWM signals to the inverter in order to turn the MOSFETs on and off. In particular, the PWM signal includes a dead time. During the dead time, the MOSFETs connected in series are simultaneously tuned off. The dead time of each PWM signal is same. That is, the dead time of each group of the MOSFETs is a same period of time.
The more the time length of the dead time in the PWM signal is increased, the less the on-period in the PWM signal to turn the MOSFET on is decreased. This means decreasing the period of outputting the available voltage or the available current. As described above, because the dead time in each PWM signal has the same value, namely has a same period of time, it is impossible to decrease the length of the dead time in the PWM signal even if one MOSFET in the switching circuit has superior response characteristics and this MOSFET allows the dead time to have a short period of time when a MOSFET in another switching circuit for another phase does not correspond to the short length of the dead time. Thus, the conventional motor control devices do not cope with, namely, do not adopt each response characteristics of the MOSFETs, and thereby makes it difficult to expand the output voltage range or the output current range of the motor.